This disclosure relates to data processing and data storage, and more specifically, to calibration of read voltage thresholds in a non-volatile memory system. Still more particularly, the disclosure relates to adaptive read voltage threshold calibration in a non-volatile memory system.
NAND flash memory is an electrically programmable and erasable non-volatile memory technology that stores one or more bits of data per memory cell as a charge on the floating gate of a transistor or a similar charge trap structure. The amount of charge on the floating gate modulates the threshold voltage of the transistor. By applying a proper read voltage and measuring the amount of current, the programmed threshold voltage of the memory cell can be determined and thus the stored information can be detected. Memories storing one, two, three and four bits per cell are respectively referred to in the art as Single Level Cell (SLC), Multi-Level Cell (MLC), Three Level Cell (TLC), and Quad Level Cell (QLC) memories. In a typical implementation, a NAND flash memory array is organized in blocks (also referred to as “erase blocks”) of physical memory, each of which includes multiple physical pages each in turn containing a multiplicity of memory cells. By virtue of the arrangement of the word and bit lines utilized to access memory cells, flash memory arrays have generally been programmed on a page basis, but erased on a block basis.
In multi-level (i.e., MLC, TLC and QLC) NAND flash memory, information is stored by programming the memory cells to various quantized threshold voltage levels according to the device's programming algorithm, which maps the binary bit values to discrete threshold voltage levels. In response to a page read command, the binary bit values are retrieved by applying appropriate read voltages that divide the programmed threshold voltage window into discrete regimes and by then applying a reverse mapping between the detected threshold voltage levels and the corresponding binary bit values. Over the lifetime of a multi-level NAND flash memory device, the distributions of programmed threshold voltage generally become degraded due to the effects of wear, retention, and read disturbs on the memory cells. Consequently, it is generally desirable to adapt or calibrate the read voltage thresholds defining the various bit values over time to compensate for the effects of wear and to extend the useful life of the NAND memory device.